1. Technical Field
The invention is related to processes for vacuum deposition of thin diamond-like carbon films on a substrate having a relatively low melting temperature such as plastic lenses.
2. Background Art
Optical lenses used in eye wear for protecting the eye from bright light fall into one of two categories. The first category includes plastic lenses such as those comprising polycarbonate or CR39 (allyl di-glycol carbonate). The advantages of this type of lens are that it is relatively light weight and does not tend to shatter under hard impact. A secondary advantage is that a polarizing material such as poly-vinyl alcohol may be readily integrated on the plastic material to enable the lens to prevent annoying reflected glare from reaching the eye. A severe disadvantage is that the plastic material is relatively soft and therefore is easily scratched, disfigured or otherwise rendered useless as an optical lens. Moreover, such plastic materials are not practically susceptible of being hardened by known processes, such as deposition of diamond-like carbon films. This is because such processes are typically carried out at high temperatures, raising the temperature of the substrate above its melting point.
A second category includes glass lenses, which enjoy the distinct advantage of being very hard and therefore virtually impervious to scratching under normal use. While the lenses made from such materials are therefore more durable and longer-lasting, they suffer from several significant disadvantages. First, glass lenses are very heavy, thus making eye-wear in which they are used less comfortable. Secondly, glass lenses tend to shatter in response to a sufficiently hard impact and they cost more than plastic lenses. Another disadvantage is that there is no practical method for providing polarizing material on a glass lens.
Accordingly there is a great need in the art for a lens which enjoys the advantages of a plastic lens as well as the advantages of a glass lens without suffering from the disadvantages of either. Such a need could be met if a way could be found to deposit a diamond-like carbon film on a plastic lens or on a lens having a plastic coating or surface. Unfortunately, there is no practical method known for depositing diamond-like carbon on such substrates.
Various processes for depositing diamond-like carbon on a substrate are known. However, none of them are disclosed as being useful with substrates such as plastic which melt at temperatures between 80.degree. C. and 150.degree. C. It is believed that in many cases such processes raise the substrate temperature well beyond 150.degree. C. and therefore cannot be used to deposit diamond-like carbon on a plastic substrate. However, as will be described later in this specification, the present invention employs relatively low temperatures of less than about 150.degree. C., preferably less than 100.degree. C. and most preferably less than about 80.degree. C., thereby allowing deposition of diamond-like carbon on substrates having lower melting temperatures.
U.S. Pat. No. 3,961,103 to Aisenberg discloses a process for depositing a diamond-like carbon film on a substrate with a glow discharge carbon ion source and an RF electric field to attractively bias the surface of the substrate while avoiding the build-up of a repulsive charge around the substrate in the presence of hydrogen gas. Aisenberg further discloses that the glow discharge carbon ion source and the substrate are held in separate evacuated chambers, the carbon ions reaching the substrate through a special constrictor electrode connecting the two chambers, the constrictor electrode having a particular arrangement with an extractor anode inside the chamber housing the substrate. This process suffers from the disadvantage of requiring two separate chambers with special electrodes to attract the carbon ions from one chamber to the other. A similar process is disclosed in Aisenberg et al. "Ion-Beam Deposition of Thin Films of Diamondlike Carbon," Journal of Applied Physics, Vol. 42, No. 7, pp. 2953-2958 (June 1971).
U.S. Pat. No. Re. 32,464 to Aine, discloses a process for depositing on a heated substrate a carbon film of a different type, namely graphite carbon from a carbon ion source. The carbon ions are generated by RF induced plasma sputtering of a carbon target electrode with argon ions from a glow discharge.
German Offenlegungsschrift 2,736,514 discloses a process for depositing a carbon film on a glass substrate using a gaseous hydrocarbon source to produce carbon ions. An RF field on the substrate attracts the carbon ions to the substrate.
Banks et al., "Ion Beam Sputter-Deposited Diamondlike Films," Journal of Vacuum Science Technology, Vol. 21, pp. 807-814 (September/October 1982) discloses a process for depositing diamond-like carbon films on a substrate by sputtering carbon from a graphite target onto the substrate while bombarding the substrate itself with argon ions. The process is directed to substrates such as silica, copper and tantalum which can endure relatively high temperatures.
U.S. Pat. No. 4,490,229 to Mertich et al. discloses a process for depositing diamond-like carbon films onto a substrate by an ion beam containing argon ions and a hydrocarbon gas such as methane while a second argon ion beam bombards the substrate which removes the lesser bound carbon atoms from the substrate.
U.S. Pat. No. 4,725,345 to Sakamoto et al. discloses a process for depositing a diamond-like carbon film onto an acoustic diaphram substrate by vaporizing carbon from a solid carbon source using an electron beam. This process suffers from the disadvantage that a D.C. (constant) field is applied to the substrate to attract the vaporized carbon, so that a repulsive surface charge accumulates on the non-conductive substrates. Therefore, the process is not suitable for use with insulating substrates such as plastic. More importantly, this process also suffers from the disadvantage that the electron beam vaporization of the solid carbon target requires a lower ambient pressure while the carbon deposition on the substrate requires a higher ambient pressure. The electron beam vaporization step must be carried out at a low ambient pressure of about 10.sup.-4 torr because at higher pressures the electron beam source is unstable due to discharge or breakdown ionization of the surrounding gas. This in turn causes the trajectory of the electron beam to be unstable. The carbon vaporization step must be carried out at a higher pressure of around 10.sup.-2 or 10.sup.-3 torr because at lower pressures there are not enough collisions between atoms and molecules in the gas atmosphere surrounding the substrate for the applied electric field to excite a plasma around the substrate. Thus, the vaporization and deposition steps appear to be incompatible for carrying out in the same vacuum chamber.
In summary, none of the foregoing processes appear to be useful for depositing diamond-like carbon films on substrates having low melting temperatures such as plastic lenses. Some of the processes involve ion beam bombardment or glow discharging which may increase the substrate temperature above the melting point of the substrate. Another process involving electron beam vaporization of a solid carbon source is problematic because its vaporization and deposition steps should be carried out at different pressures and are therefore incompatible for use in the same vacuum chamber.